<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <input     id="num1" />
    +
    <input   id="num2"  />
    <input  type="button"  value="="    onclick="jisuan()"  />
    <input   id="num3"  />
    <script>
        function  jisuan(){
        num3.value= parseInt(num1.value)+parseInt(num2.value);
        }



        </script>

</body>
</html>